Steel form

ABSTRACT

A steel form is a steel form for forming a binding beam, including: a pair of Z-steels, wherein each of the pair of Z-steels is provided with a bottom plate portion and a side plate portion extending upward from the bottom plate portion, the bottom plate portion has a joining surface for joining the respective bottom plate portions of the pair of Z-steels to each other, and a groove portion allowing concrete placement is formed by the bottom plate portion and the side plate portion of each of the pair of Z-steels

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Patent Application inJapan No. 2017-036750 filed on Feb. 28, 2017 and the benefit of PCTapplication No. PCT/JP2018/005971 filed on Feb. 20, 2018, the disclosureof which is incorporated by reference its entirety.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference in their entirety to the sameextent as if each individual publication or patent application wasspecifically and individually indicated to be incorporated by reference.

TECHNICAL FIELD

The present invention relates to a steel form.

BACKGROUND ART

Proposed in the related art is a method for using a grooved steel form(approximately U-shaped in cross section) matching the outer shell shapeof a beam as a beam form for labor-saving form construction for example,Patent Document 1).

CITATION LIST Patent Document

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. Heisei 10-140654

SUMMARY OF THE INVENTION Technical Problem

The steel form described in Patent Document 1 needs to be formed byprocessing a thin steel plate into a shape matching the outer shellshape of a beam by, for example, roll molding or press molding. As aresult, it takes labor and cost to process the steel form. Besides, thegrooved steel form is bulky in terms of transport, and thus the numberof the grooved steel forms that can be transported at one time islimited. As a result, it takes labor and cost to transport the steelform.

It is an object of the present invention to solve the problems of theabove mentioned prior arts.

Means for Solving the Problems

One aspect of the present invention provides a steel form, which is asteel form for steel-framed concrete beam formation, includes a pair offrame members, wherein each of the pair of frame members is providedwith a bottom plate portion and a side plate portion extending upwardfrom the bottom plate portion, the bottom plate portion has a joiningsurface for joining the respective bottom plate portions of the pair offrame members to each other, and a groove portion allowing concreteplacement is formed by the respective bottom and side plate portions ofthe pair of frame members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are a set of views illustrating a steel-framedconcrete beam (binding beam) according to Embodiment 1 of the invention,in which FIG. 1(a) is a left side view and FIG. 1(b) is across-sectional view taken along arrow A-A in FIG. 1(a).

FIG. 2 is an exploded perspective view illustrating a temporary stateduring construction in the vicinity of the joining portion between thebinding beam and a girder.

FIG. 3 is a view illustrating the relationship between a cross sectionof the binding beam and calculation parameters.

FIG. 4 is a graph showing the relationship between a slab thickness anda long-term bending rigidity ratio.

FIG. 5 is a graph showing the relationship between the slab thicknessand a short-term bending rigidity ratio.

FIG. 6 is a graph showing the relationship between the load that isapplied to the binding beam and the shear rigidity ratio of a steelform, which pertains to a case where no web opening is present.

FIG. 7 is a graph showing the relationship between the load that isapplied to the binding beam and the shear rigidity ratio of the steelform, which pertains to a case where a web opening is present.

FIGS. 8(a)-8(c) are a set of cross-sectional perspective viewscorresponding to the A-A arrow cross section in FIG. 1(a), in which FIG.8(a) illustrates the binding beam at the completion of a steel forminstallation step, FIG. 8(b) illustrates the binding beam at thecompletion of main bar arrangement, deck plate installation, andplacement steps, and FIG. 8(c) illustrates the binding beam at thecompletion of a penetration step.

FIGS. 9(a)-9(c) are a set of cross-sectional perspective viewscorresponding to the A-A arrow cross section in FIG. 1(a), in which FIG.9(a) illustrates the binding beam at the completion of steel forminstallation and cylindrical form installation steps, FIG. 9(b)illustrates the binding beam at the completion of main bar arrangement,deck plate installation, and placement steps, and FIG. 9(c) illustratesthe binding beam at the completion of a penetration step.

FIGS. 10(a) and 10(b) are a set of views illustrating a state where aZ-steel is transported, in which FIG. 10(a) is an end view illustratingthe state of transport of a Z-steel of Embodiment 1 and FIG. 10(b) is anend view illustrating the state of transport of a Z-steel according to afirst modification example.

FIGS. 11(a) and 11(b) are a set of views illustrating a steel formaccording to a second modification example, in which FIG. 11(a) is aplan view of the steel form that is yet to be bent and FIG. 11(b) is aside view of the steel form that is bent.

FIGS. 12(a) and 12(b) are a set of views illustrating the vicinity ofthe joining portion between a binding beam and a girder according to athird modification example, in which FIG. 12(a) is a left side view andFIG. 12(b) is a cross-sectional view taken along arrow B-B in FIG.12(a).

FIGS. 13(a) and 13(b) are a set of views illustrating the vicinity ofthe joining on between a binding beam and a girder according to a fourthmodification example, in which FIG. 13(a) is a right side view and FIG.13(b) is a plan view.

FIG. 14 is a right side view illustrating the vicinity of the joiningportion between a binding beam and a girder according to a fifthmodification example.

FIG. 15 is a right side view illustrating the vicinity of the joiningportion between a binding beam and a girder according to a sixthmodification example.

FIG. 16 is a perspective view of an end portion of the steel form of thebinding beam in FIG. 15.

FIG. 17 is a right side view illustrating the vicinity of the joiningportion between a binding beam and a girder according to a seventhmodification example.

FIG. 18 is a side view illustrating the vicinity of the joining portionbetween a binding beam and a girder according to an eighth modificationexample.

FIG. 19 is a plan view of FIG. 18.

FIG. 20 is a cross-sectional view corresponding to the A-A arrow crosssection in FIG. 1(a) and is a cross-sectional view of a steel form of abinding beam according to a ninth modification example.

FIG. 21 is a cross-sectional view corresponding to the A-A arrow crosssection in FIG. 1(a) and is a cross-sectional view of a steel form of abinding beam according a tenth modification example.

FIGS. 22(a) and 22(b) are a set of cross-sectional views correspondingto the A-A arrow cross section in FIG. 1(a), in which FIG. 22(a)illustrates a steel form of a binding beam according to an eleventhmodification example and FIG. 22(b) illustrates a steel form of abinding beam according to a twelfth modification example.

FIGS. 23(a) and 23(b) are a set of cross-sectional views correspondingto the A-A arrow cross section in FIG. 1(a), in which FIG. 23(a)illustrates a steel form of a binding beam according to a thirteenthmodification example and FIG. 3(b) illustrates a steel form of a bindingbeam according to a fourteenth modification example.

DESCRIPTION OF EMBODIMENTS

Embodiments of a steel form according to the invention will be describedin detail below with reference to accompanying drawings. The basicconcepts of the embodiments ([I]) will be described first, and thendetails of the embodiments ([II]) will be described. Modificationexamples regarding the embodiments ([III]) will be described last. Theinvention is not limited by the embodiments.

[I] Basic Concepts of Embodiments

The basic concepts of the embodiments will be described first.

The embodiments relate to a steel form. The “steel form” is a formprovided with steel for forming steel-framed concrete beams constitutingbuilding. The “steel-framed concrete beam” is a beam provided with atleast a steel frame and concrete. The steel-framed concrete beam may beprovided with a component other than the steel frame and the concrete.For example, the embodiments illustrate an example in which thesteel-framed concrete beam is configured as a steel-framed reinforcedconcrete beam that has a rebar in addition to a steel frame andconcrete. Although the steel-framed reinforced concrete beam may beprovided with, for example, a main bar and a stirrup as the rebar, acase where the steel-framed reinforced concrete beam is provided with amain bar and no stirrup will be described below. The steel-framedconcrete beam may be provided with, for example, a stirrup and no mainbar, both a main bar and a stirrup, or no main bar and no stirrup.

The steel frame is capable of having any shape insofar as the steelframe functions as a form allowing concrete placement. A case where thesteel frame has an axial cross section in a hat shape (a shape obtainedby joining a pair of Z-steels to each other) will be described below.

The steel-framed concrete beam according to the embodiments isapplicable to any installation floor. Although a case where thesteel-framed concrete beam is a second floor beam will be describedbelow, the steel-framed concrete beam is applicable to beams of otherfloors as well. Although a case where the steel-framed concrete beam isa binding beam will be described below, the steel-framed concrete beammay be a girder as well.

[II] Details of Embodiments

Details of the embodiments will be described below

Embodiment 1

The steel-framed concrete beam according to Embodiment 1 will bedescribed first.

Configuration

FIG. 1 is a set of views illustrating the steel-framed concrete beamaccording to Embodiment 1 (hereinafter, simply referred to as “bindingbeam” 1). FIG. 1(a) is a left side view and FIG. 1(b) is across-sectional view taken along arrow A-A in FIG. 1(a). As illustratedin FIG. 1, the binding beam 1 according to Embodiment 1 is provided witha steel form 10, binding beam concrete 20, main bars 30, and webopenings (through holes) 40. In the following description, the +X-Xdirection in each drawing will be referred to as “width direction” asnecessary. In particular, the +X direction will be referred to as“rightward direction” and the −X direction will be referred to as“leftward direction”. The +Y-Y direction will be referred to as “depthdirection” or “forward-rearward direction”. In particular, the +Ydirection will be referred to as “forward direction” and the −Ydirection will be referred to as “rearward direction”. The +Z-Zdirection will be referred to as “height direction” or “upward-downwarddirection”. In particular, the +Z direction will be referred to as“upward direction” and the −Z direction will be referred to as “downwarddirection”. As for a vertical plane (YZ plane) passing through the axialcenter of the steel-framed concrete beam, the direction toward the planealong the width direction (+X-X) will be referred to as “inwarddirection” and the direction away from the plane along the widthdirection (+X-X) will be referred to as “outward direction”.

Configuration-Steel Form

The steel form 10 is a steel form that has a groove portion (describedlater) for placing the binding beam concrete 20. This steel form 10 isprovided in each binding beam 1 constituting a building and is disposedso as to cover the binding beam 1 from below. As illustrated in thedrawing, the steel form 10 of Embodiment 1 is formed by a pair of (thatis, two) Z-steels 11 being mutually joined in bottom plate portions 12(described later) at a construction site. The invention is not limitedthereto, and the steel form 10 may be integrally formed with a singlemember or may be formed by three or more members being combined. In acase where three or more members are combined as described above, forexample, the integrally formed members (the bottom plate portion 12, aside plate portion 13, a flange portion 14, and a reinforcing portion 15to be described later) that constitute the Z-steel 11 may be formedseparately. Each of the pair of Z-steels 11 can be substantially similarin configuration to the other, and thus only one of the Z-steels 11 willbe described below. In a case where the Z-steels 11 need to bedistinguished from each other, the Z-steel 11 that is positioned on theright of the binding beam 1 (in the +X direction) will be referred to as“right Z-steel” and the Z-steel 11 that is positioned on the left of thebinding beam 1 (in the −X direction) will be referred to as “leftZ-steel”. A specific method for forming the steel form 10 will bedescribed later.

The Z-steel 11 is a frame member that constitutes the steel form 10. Asillustrated in FIG. 1(b), the Z-steel 11 is a steel material that has asubstantially Z-shaped axial cross section. The Z-steel 11 is providedwith the bottom plate portion 12, the side plate portion 13, the flangeportion 14, and the reinforcing portion 15.

The bottom plate portion 12 is a steel plate positioned on the bottomsurface of the steel form 10. The bottom plate portion 12 has a joiningsurface 16 for mutually joining the respective bottom plate portions 12of the pair of Z-steels 11. The pair of Z-steels 11 are joined to eachother on the joining surface 16. For example, in Embodiment 1, a part ofthe bottom plate portion 12 of the right Z-steel is superposed on a partof the bottom plate portion 12 of the left Z-steel and each of the partswhere the pair of Z-steels 11 are in contact with each other (the uppersurface of the bottom plate portion 12 of the left Z-steel and the lowersurface of the bottom plate portion 12 of the right Z-steel) is thejoining surface 16. The joining on the joining surface 16 can beperformed by any specific method. For example, in Embodiment 1, aplurality of bolt holes (not illustrated) are spaced apart along thelongitudinal direction (+Y-Y direction) of the beam in the joiningsurfaces 16 of both Z-steels 11 and both Z-steels 11 are joined by boltfastening by means of the bolt holes. Specific joining methods are notlimited thereto. For example, welding-based joining and screwpenetration-based joining may be performed instead.

The side plate portion 13 is a steel plate extending in the upwarddirection from the bottom plate portion 12. Specifically, the side plateportion 13 is a part that is folded back from the outer end of thebottom plate portion 12 and extends to the upper end of the beam and ispositioned so as to cover the left and right sides of the binding beam1. The length of the side plate portion 13 in the height direction (+Z-Zdirection) is longer, by the thickness of the bottom plate portion 12,in the left Z-steel than in the right Z-steel. This is for the upper endpositions of the side plate portions 13 of both Z-steels 11 (that is,the height positions of the flange portions 14) to coincide with eachother when the pair of Z-steels 11 are overlapped.

In the following description, the part that is formed by the side plateportions 13 and the bottom plate portions 12 of a pair of the steelforms 10 and has a U-shaped axial cross section will be referred to asgroove portion as necessary. Concrete can be placed in the grooveportion by the steel form 10 forming the groove portion as describedabove. The lower and side parts of the binding beam 1 are covered with asteel plate by the groove portion, and thus it is possible to detersteam from escaping from the lower and side parts of the binding beamconcrete 20 during a fire, it is possible to deter a temperature rise inthe room below the binding beam 1, and it is possible to improve thefire resistance performance of the binding beam 1.

The flange portion 14 is a steel plate extending in the outwarddirection from the upper end of the side plate portion 13. Specifically,the flange portion 14 is a part that is folded back in the outwarddirection from the upper end of the side plate portion 13 and extendsalong a horizontal plane, and a deck plate 3 is placed and screwed onthe flange portion 14. Although a case where this deck plate 3 is aknown corrugated steel plate will be described, the invention is notlimited thereto and a flat plate may be used as the deck plate 3.Although illustration is omitted, the binding beams 1 are arranged sideby side at intervals along the longitudinal direction of a girder 2, oneend portion of the deck plate 3 is placed in the flange portion 14 ofone binding beam 1 as illustrated in FIG. 1(b), and the other endportion of the deck plate 3 is similarly placed in the flange portion 14of the binding beam 1 that is adjacent to the one binding beam 1. By theflange portion 14 being provided as described above, the load of slabconcrete 4 (described later) can be received by the flange portion 14and is allowed to smoothly flow to the binding beam 1 and the proofstress of the binding beam 1 is improved.

The reinforcing portion 15 is a steel plate extending in the downwarddirection from the outer end of the flange portion 14. By thereinforcing portion 15 being provided as described above and thicknessbeing given to the outer end of the flange portion 14, the localbuckling of the outer end of the flange portion 14 that pertains to acase where the slab concrete 4 is placed and the flange portion 14receives the load of the slab can be deterred. In addition, it ispossible to reduce the overall thickness of the steel form 10 by locallyreinforcing only a low-strength part by means of the reinforcing portion15. The reinforcing portion 15 of Embodiment 1 extends in the downwarddirection from the outer end of the flange portion 14. The invention isnot limited thereto and the reinforcing portion 15 may extend in, forexample, the upward direction.

Configuration-Binding Beam Concrete

The binding beam concrete 20 is concrete placed in the groove portionthat the pair of side plate portions 13 and the bottom plate portion 12of the steel form 10 constitute. The binding beam concrete 20 is knownconcrete solidified after filling in the groove portion, and theplurality of web openings 40 are formed in the binding beam concrete 20as described above. The slab concrete 4 for forming an upper floor slabis formed along a horizontal plane above the binding beam concrete 20.Girder concrete (reference numeral omitted) for forming the girder 2 isformed, so as to be orthogonal to the binding beam 1, at the front andrear ends of the binding beam concrete 20. Although the binding beamconcrete 20, the slab concrete 4, and the girder concrete are givendifferent names and reference numerals, the binding beam concrete 20,the slab concrete 4, and the girder concrete are simultaneously placedand formed in Embodiment 1. The binding beam concrete 20, the slabconcrete 4, and the girder concrete will be simply referred to as“concrete” when no distinguishment among them is necessary.

Configuration-Main Bar

The main bars 30 are rebars extending along the axial center directionof the beam. Although two upper end bars and four lower end bars areillustrated as an example in Embodiment 1, the number and disposition ofthe main bars 30 are not limited thereto.

Configuration-Web Opening

The web opening 40 is a hole formed so as to penetrate the side plateportion 13 and the binding beam concrete 20. The web opening 40 isformed by, for example, the side plate portion 13 and the binding beamconcrete 20 being drilled with a drill after the binding beam concrete20 placed in the steel form 10 is solidified. By the web opening 40being formed as described above, a duct or piping for air conditioning,electrical equipment, and so on can be passed through the web opening 40(a case where a duct for air conditioning is passed through the webopening 40 will be described below Accordingly, the duct can be extendedfrom one of spaces sandwiching the beam 1 (such as the space to theright of the binding beam 1) to the other thereof (such as the space tothe left of the binding beam 1) and the degree of freedom of ductdisposition is improved.

The web opening 40 is formed in the web opening forming portion of thebinding beam 1. The “web opening forming portion” is a part where theweb opening 40 penetrating the side plate portion 13 and the bindingbeam concrete 20 can be formed. Specifically, the “web opening formingportion” is a part where no rebar (main bar 30 in Embodiment 1) isarranged (part where the drill does not interfere with the rebar whenthe web opening 40 is drilled with the drill). For example, inEmbodiment 1, the “web opening forming portion” is a part above thelower main bar 30 (lower end bar) in the binding beam 1. The number ofthe web openings 40 is six and the web openings 40 are along the axialcenter direction of the beam in the illustration. The number of the webopenings 40 is not limited to six.

Configuration-Girder Joining Portion

The joining portion between the binding beam 1 and the girder 2according to Embodiment 1 will be described below. FIG. 2 is an explodedperspective view illustrating a temporary state during construction inthe vicinity of the joining portion between the binding beam 1 and thegirder 2. The concrete and the rebar that constitute the binding beam 1and the girder 2 are not illustrated in FIG. 2 for convenience ofillustration. As illustrated in FIG. 2, a notch (hereinafter, referredto as binding beam accommodating portion 2 b) having a shape (hat shape)substantially corresponding to the axial cross-sectional shape of thebinding beam 1 is formed in a side surface of a wooden form 2 a of thegirder 2 according to Embodiment 1. The binding beam 1 and the girder 2can be formed at the same time by concrete being simultaneously placedin the wooden form 2 a of the girder 2 and the steel form 10 with thesteel form 10 of the binding beam 1 fitted in the binding beamaccommodating portion 2 b. As illustrated in the drawing, notches(hereinafter, referred to as flange accommodating portions 2 c) havingthe same width as the flange portion 14 are formed on the left and rightof the upper end of the binding beam accommodating portion 2 b. Theflange portion 14 can be housed in the flange accommodating portion 2 c.In a case where the flange portion 14 is housed in the flangeaccommodating portion 2 c as described above, a gap equivalent to theheight of the reinforcing portion 15 is formed below the flange portion14. A sealing material 2 d (illustrated rectangular wood or the like)filling this gap is disposed for prevention of concrete leakage from thegap.

Temporary supports (not illustrated) may support the binding beam 1until concrete placement. The positions and number of the temporarysupports may be appropriately changed in accordance with the length andweight of the binding beam 1. For example, one temporary support may beprovided in one axial end portion, one temporary support may be providedin the other axial end portion, and one temporary support may beprovided in the axial middle portion. The steel form 10 is higher inproof stress than the wooden form 2 a, and thus the temporary supportsmay be omitted if the temporary supports are unnecessary in view of thelength and weight of the binding beam 1.

Method for Designing Steel Form

Next, an example of a method for designing the steel form 10 accordingto Embodiment 1 will be described. In the present embodiment, theallowable bending moment or the allowable shear force of the bindingbeam 1 is calculated by the following Equation (1).

F _(a) =F _(RC) +β·F _(S)   (Equation 1)

F_(a): allowable bending moment or allowable shear force of binding beam1

F_(RC): allowable bending moment or allowable shear force of bindingbeam concrete 20 (hereinafter, referred to as reinforced concrete (“RC”)as necessary)

β: burden factor of allowable bending moment or allowable shear force ofsteel form 10, which is 0.5 or less

F_(S): allowable bending moment or allowable shear force of steel form10

Method for Designing Steel Form-Method for Designing Allowable BendingMoment

This design method will be divided into an allowable bending momentdesign method and an allowable shear force design method and describedin further detail below. The allowable bending moment design method willbe described first. The allowable bending moment is designed afterdivision into a long-term allowable bending moment and a short-termallowable bending moment. The long-term allowable bending moment iscalculated by the following Equation (2). The short-term allowablebending moment is calculated by the following Equation (3). FIG. 3 is aview illustrating the relationship between the cross section of thebinding beam 1 and calculation parameters.

_(L) M _(a)=_(L) M _(RC)+_(L)β_(M)·_(L) M _(S)   (Equation 2)

_(S) M _(a)=_(S) M _(RC)+_(S)β_(M)·_(S) M _(S)   (Equation 3)

_(L)M_(RC): long-term allowable bending moment of RC cross section part(which may be a_(t)·_(L)f_(t)·j in case where tensile rebar ratio of RCcross section is balanced rebar ratio or less)

_(S)M_(RC): short-term allowable bending moment of RC cross section part(which may be a_(t)·_(S)f_(t)·j in case where tensile rebar ratio of RCcross section is balanced rebar ratio or less)

a_(t): tensile rebar cross-sectional area

_(L)f_(t): long-term allowable tensile stress of tensile rebar

_(S)f_(t): short-term allowable tensile stress of tensile rebar

j: stress center distance (j=(⅞)·d)

d: effective depth of cross section (distance from upper surface ofbinding beam 1 to concrete bar arrangement)

_(L)β_(M): long-term steel frame bending burden effective factor of 0.5or less, 0.1 here

_(S)β_(M): short-term steel frame bending burden effective factor of 0.5or less, 0.4 here

_(L)M_(S): long-term allowable bending moment of S cross section part(_(L)M_(S)=_(L)σ_(t)·Z_(S))

_(S)M_(S): short-term allowable bending moment of S cross section part(_(S)M_(S)=_(S)σ_(t)·Z_(S))

_(L)σ_(t): long-term allowable tensile stress of steel form 10

_(S)σ_(t): short-term allowable tensile stress of steel form 10

Z_(S): section modulus of steel form 10

An ultimate bending strength Mu is calculated by the following Equation(4),

M _(u) =M _(uRC) +M _(uS)   (Equation 4)

M_(uRC): ultimate bending strength of RC cross section part(M_(uRC)=0.9·a_(t)·1.1·_(S)f_(t)·d)

a_(t): tensile rebar cross-sectional area

_(S)f_(t): short-term allowable tensile stress of tensile rebar

d: effective depth of cross section

M_(uS): ultimate bending strength of S cross section part(M_(uS)=1.1·_(S)σ_(t)·Z_(p))

_(S)σ_(t): short-term allowable tensile stress of steel form 10

Z_(p): plastic section modulus of steel form 10

The long-term allowable bending moment is an allowable bending momentover a relatively long time (such as several years to several decades).The short-term allowable bending moment is an allowable bending momentover a relatively short time (such as several hours to several days).The allowable bending moment is calculated after the division into thetwo periods as described above so that an allowable bending momentsuitable for each load bearing ratio is designed in view of the factthat the load bearing ratio of the RC and the steel form 10 in thebinding beam 1 can vary as the situation of loading on the binding beam1 can vary with the lengths of the periods. In other words, it isassumed that the loading on the binding beam 1 is relatively small in arelatively long time, and thus it is assumed that the RC of the bindingbeam 1 is maintained without breaking (see the lower left cross sectionin FIG. 4 (described later)) and the load bearing ratio of the RCincreases. In a relatively short time, it is assumed that the loading onthe binding beam 1 is relatively large (for example, the loading becomesrelatively large by a forklift that carries a heavy object passingthrough the binding beam 1), and thus it is assumed that the loadbearing ratio of the RC decreases as a result of cracking at the lowerend of the RC of the binding beam 1 (see the lower left cross section inFIG. 5 (described later), as indicated by a diagonal line in this crosssection, it is assumed that only approximately the upper two-thirds ofthe slab part of the RC remains without cracking and bears the load). Inthis regard, in the present embodiment, the load bearing ratio of the RCand the steel form 10 in the binding beam 1 is expressed in Equations 2and 3 as a steel frame bending burden effective factor β_(M), and thenthis steel frame bending burden effective factor β_(M) is givendifferent values in the long-term and short-term cases and the allowablebending moment suitable for each load bearing ratio is designed as aresult. By adopting the design method, it is possible to calculate acomplex allowable bending moment taking long-term and short-term loadingsituations into account and it is possible to optimize the design of thebinding beam 1.

The steel frame bending burden effective factor β_(M) can be calculatedfrom a bending rigidity ratio ζ_(M)(=E_(S)I_(S)/E_(C)I_(C)) of the RCand a bending rigidity E_(S)I_(S) of the steel form 10. The bendingrigidity ratio ζ_(M) can vary with the plate thickness of the steel form10 and the thickness of the slab concrete 4 attached to the binding beam1 (hereinafter, referred to as “slab” as necessary), and thus anapplication restriction range is set for each of the plate thickness ofthe steel form 10 and the thickness of the slab, the bending rigidityratio ζ_(M) is calculated on the premise of the application restrictionrange, and the steel frame burden effective factor β_(M) is determinedfrom the calculated bending rigidity ratio ζ_(M). Specifically, theplate thickness of the steel form 10 has an application restrictionrange of 3.2 mm or more. The load bearing ratio of the steel form 10increases as the plate thickness of the steel form 10 increases, andthus a lower limit value of “3.2 mm” and application restriction rangesetting “at or above” the lower limit value allow the steel frame burdeneffective factor β_(M) to remain above it insofar as the plate thicknessof the steel form 10 is determined in the application restriction range.The thickness of the slab has an application restriction range of 200 mmor less. The ratio of load bearing by the slab increases as thethickness of the slab increases, and then the load bearing ratio of thesteel form 10 decreases. Accordingly, an upper limit value of “200 mm”and application restriction range setting “at or below” the upper limitvalue allow the steel frame burden effective factor β_(m) to remainabove it insofar as the thickness of the slab is determined in theapplication restriction range.

FIG. 4 is a graph showing the relationship between the thickness of theslab and a long-term bending rigidity ratio _(L)ζ_(M), and FIG. 5 is agraph showing the relationship between the thickness of the slab and ashort-term bending rigidity ratio _(S)ζ_(M). In each graph, thehorizontal axis represents the thickness of the slab, the vertical axisrepresents the bending rigidity ratio ζ_(M) (long-term bending rigidityratio _(L)ζ_(M) or short-term bending rigidity ratio _(S)ζ_(M)), thesolid line indicates a load of 3.2 tons, and the dotted line indicates aload of 4.5 tons. It is assumed that the cross-sectional shape of thebinding beam 1 is a standard cross section (6.5 in in total length, 300mm in total width, and 550 mm in total height). As shown in FIG. 4, inthe long term, the long-term bending rigidity ratio _(L)ζ_(M) isapproximately 0.12 at 200 mm, which is the upper limit value of theapplication restriction range of the slab thickness, and thus thelong-term bending rigidity ratio _(L)ζ_(M) was set to 0.1 in view ofsafety. As shown in FIG. 5, in the short term, the short-term bendingrigidity ratio _(S)ζ_(M) is approximately 0.49 at 200 mm, which is theupper limit value of the application restriction range of the slabthickness, and thus the short-term bending rigidity ratio _(S)ζ_(M)wasset to 0.4 in view of safety. Then, calculation can be performed basedon the long-term bending rigidity ratio _(L)ζ_(M) of 0.1 and theshort-term bending rigidity ratio _(S)ζ_(M) of 0.4 and in accordancewith proof stress formula M_(a)=(1+_(L)ζ_(M))M_(RC) and steel framebending burden effective factor β_(M)=ζ_(M)(M_(RC)/M_(S)). Here,M_(RC)/M_(S) is the allowable proof stress ratio between the RC crosssection and the steel form 10 and M_(RC)/M_(S) is 1.35 in a case wherethe bar arrangement of the RC cross section is 4-HD13 (four deformedrebars (steel deformed bars) having a yield point of 345 N/mm2 or more)and the plate thickness of the steel form 10 is 3.2 mm in the crosssections in FIGS. 4 and 5. Here, the steel frame bending burdeneffective factor pm was calculated using M_(RC)/M_(S)=1.0 as a value ofsafety side. As described above, in the present embodiment, a simplifiedmethod (β method) is used in which the restriction of the applicationrestriction range is applied to the plate thickness of the steel form 10and the thickness of the slab. Alternatively, a detail method (ζ method)may be adopted in which the bending rigidity ratio ζ_(M) is set inaccordance with each cross-sectional shape (plate thickness of the steelform 10, slab thickness, and bar arrangement) and the steel framebending burden effective factor β_(M) is calculated by using proofstress formula M_(a)=(1+_(L)ζ_(M))M_(RC). Here, the design formula isdetermined on the safety side so that the design formula does not becomecomplicated (low iron burden rate being set in terms of design). Asconfirmed by the inventor's experiment, the cross section part of thesteel form 10 is restrained by the RC cross section part and the steelform 10 undergoes no lateral buckling as a thin plate, and thus atensile stress f_(t) is adopted as an allowable stress f_(b) of thesteel material of the steel form 10.

Method for Designing Steel Form-Method for Designing Allowable ShearForce

Next, the allowable shear force design method will be described. Theallowable shear force is designed after division into a long-termallowable shear force and a short-term allowable shear force similarlyto the above idea related to the allowable bending moment. The long-termallowable shear force is calculated by the following Equation (5) andthe short-term allowable shear force is calculated by the followingEquation (6). The relationship between the cross section of the bindingbeam 1 and calculation parameters is as illustrated in FIG. 3.

_(L) Q _(a) =α·A _(C)·_(L) f _(S)+β_(Q)·_(S) A _(W)·_(L)σ_(S)  (Equation 5)

_(S) Q _(a) =α·A _(C) _(S) f _(S)+β_(Q)·_(S) A _(W)·_(S)σ_(s)  (Equation 6)

α: additional factor by shear span ratio (M/Q_(d))

A_(C): shear effective cross-sectional area of RC portion(A_(C)=B·j+2·B₂·t)

_(L)f_(s): long-term allowable shear stress of concrete

_(S)f_(S): short-term allowable shear stress of concrete

β_(Q): steel frame shear burden effective factor of 0.5 or less, 0.2here

_(S)A_(W): shear cross-sectional area of steel form 10(_(S)A_(W)=2·t_(S)·(H−2·r))

t_(S): thickness of steel plate

r: curvature radius of corner of Z-steel plate 11

_(L)σ_(S): long-term allowable shear stress of steel material of Z-steelplate 11 (_(L)σ_(S)=square root of _(L)σ_(t)/3)

_(S)σ_(S): short-term allowable shear stress of steel material ofZ-steel plate 11 (_(S)σ_(S)=square root of _(S)σ_(t)/3)

The shear effective cross-sectional area Ac of the RC portion used inthe shear force calculation is the same cross section as the bindingbeam 1 used in the experiment as illustrated in FIG. 3 and thecross-sectional area of the slab on the flange portion of the steel form10 may also be included. The steel frame shear burden effective factorβ_(Q) in the shear force calculation formula can be obtained from ashear rigidity ratio ζ_(Q) of the steel form 10 indicated by the resultof the inventor's experiment. FIG. 6 is a graph showing the relationshipbetween the load that is applied to the binding beam 1 and the shearrigidity ratio ζ_(Q) of the steel form 10, which pertains to a casewhere the web opening (opening) 40 is absent. FIG. 7 is a graph showingthe relationship between the load that is applied to the binding beam 1and the shear rigidity ratio ζ_(Q) of the steel form 10, which pertainsto a case where the web opening (opening) 40 is present. In each graph,the horizontal axis represents the applied load and the vertical axisrepresents the shear rigidity ratio ζ_(Q). As is apparent from FIGS. 6and 7, the shear rigidity ratio ζ_(Q) of the steel form 10 issubstantially constant at approximately 0.2 regardless of the presenceor absence of the web opening 40 and the magnitude of the applied load.Accordingly, in the present embodiment, the steel frame shear burdeneffective factor β_(Q) was obtained with shear rigidity ratio ζ_(Q) setto 0.2. The steel frame shear burden effective factor β_(Q) iscalculated from β_(Q)=ζ_(Q)(Q_(RC)/Q_(S)) by detail method (ζmethod)-based proof stress formula Q_(L)=(1+_(L)ζ_(Q))_(L)Q_(RC). Here,Q_(RC)/Q_(S) is the ratio between the shear capacity of the steel form10 and the RC cross section. Q_(RC)/Q_(S) is 1.04 in a case where thecross-sectional shape of the binding beam 1 is a standard cross section(6.5 m in total length, 300 mm in total width, and 550 mm in totalheight) and the thickness of the steel form 10 is 3.2 mm. Here, thesteel frame shear burden effective factor β_(Q) of 0.2 was calculatedusing Q_(RC)/Q_(S)=1.0 as a value of safety side. Also in this sheardesign formula, ζ_(Q) is constant at 0.2 as the detail method (ζ method)and obtainment is also possible from equation Q_(a)=(1+ζ_(Q))Q_(RC)obtained from the allowable proof stress of the RC cross section. Aswith the bending design formula, however, the steel frame burdeneffective factor was clarified in the design formula.

As described above, the long-term steel frame bending burden effectivefactor _(L)β_(M) is 0.1 and the short-term steel frame bending burdeneffective factor _(S)β_(M) is 0.4 in the design of the allowable bendingmoment. In the design of the allowable shear force, the steel frameshear burden effective factor β_(Q) is 0.2. Although the burden factor βof the steel form 10 may be another value, the upper limit of the loadbearing ratio of the steel form 10 is set to 50% and the burden factor βof the steel form 10 is set to 0.5 or less for safety enhancement. Thelower limit of the load bearing ratio of the steel form 10 can be atleast 10% in view of the graphs in FIGS. 6 and 7 and the burden factor βof the steel form 10 can be set to 0.1 or more. However, the steel form10 may be used only as a form of the binding beam concrete 20 and thesteel form 10 may be allowed to bear no load. In this case, the burdenfactor β of the steel form 10 may be 0. By adopting the design method,it is possible to calculate a complex allowable bending moment and acomplex allowable shear force taking the respective bearing ratios ofthe steel form 10 and the binding beam concrete 20 into account and itis possible to optimize the design of the binding beam 1.

Steel Form Forming Method

Next, an example of the method for forming the steel form 10 accordingto Embodiment 1 will be described. First, the Z-steel 11 is manufacturedat a factory. The Z-steel 11 can be manufactured by any specific method.For example, the Z-steel 11 can be formed by bending of one flat thinsteel plate. Subsequently, the manufactured Z-steel 11 is transported toa construction site. At this time, a plurality of the Z-steels 11 can betransported in an overlapping manner, and thus it is possible totransport more Z-steels 11 at one time than in the case of transportingthe pair of mutually joined Z-steels 11. Transport efficiencyenhancement can be achieved as a result.

The sealing material (small piece) 2 d described with reference to FIG.2 may be attached to the lower part of the flange portion 14 by anymethod such as adhesion before the transport. In this case, the strengthof the flange portion 14 or the reinforcing portion 15 can be enhancedby the sealing material 2 d and deformation of the flange portion 14 orthe reinforcing portion 15 attributable to a load or an impact duringthe transport can be prevented. For a similar purpose, a reinforcingmaterial (not illustrated) similar in shape to the sealing material 2 dmay be provided at a predetermined interval below the flange portion 14or a long reinforcing material (not illustrated) resulting fromextension of the sealing material 2 d in the Y direction in FIG. 2 maybe provided below the flange portion 14. Such reinforcing materials maybe removed after the transport or may be permanently fixed withoutremoval. The strength of the flange portion 14 or the reinforcingportion 15 can be reduced to the same extent in a case where thestrength of the flange portion 14 or the reinforcing portion 1.5 can beimproved by such a reinforcing material being provided, and thus theflange portion 14 and the reinforcing portion 15 may be reduced inthickness or the dimension at which the reinforcing portion 15 extendsfrom the flange portion 14 may be shortened.

Next, the pair of Z-steels 11 transported to the construction site arejoined together and the steel form 10 is formed. Specifically, asillustrated in FIG. 1(b), the bottom plate portions 12 of the rightZ-steel and the left Z-steel are overlapped and, in that state, a boltmay be inserted through and fastened in each of bolt holes illustrated)formed at an appropriate interval at the overlapping part of both bottomplate portions 12. When both Z-steels are joined in this manner, it ispreferable to attach a member for maintaining a constant intervalbetween the respective side plate portions 13 of the Z-steels 11. Forexample, a batten positioned in the groove portion and fixing theinterval by propping the side plate portions 13 or a U-shaped veneerboard fitted to the outer edge shape of the groove portion may betemporarily installed and removed after both Z-steels 11 are joined toeach other.

Binding Beam Construction Method

A method for constructing the binding beam 1 according to Embodiment 1will be described below. FIG. 8 is a set of cross-sectional perspectiveviews corresponding to the A-A arrow cross section in FIG. 1(a). FIG.8(a) illustrates the binding beam 1 at the completion of a steel forminstallation step. FIG. 8(b) illustrates the binding beam 1 at thecompletion of main bar arrangement, deck plate installation, andplacement steps, FIG. 8(c) illustrates the binding beam 1 at thecompletion of a penetration step.

First, the steel form installation step is performed as illustrated inFIG. 8(a). In the steel form installation step, the steel form 10 formedby the above-described forming method is lifted by a heavy machine orthe like and installed at a beam construction position. In Embodiment 1,the installation is performed such that an end portion of the steel form10 is connected to the wooden form 2 a of the girder 2 as illustrated inFIG. 2. For convenience of illustration, the steel form 10 of thebinding beam 1 in FIG. 2 is illustrated as being tightly fit in thenotch (binding beam accommodating portion 2 b) of the wooden form 2 a ofthe girder 2. However, the invention is not limited thereto. The bindingbeam accommodating portion 2 b may be enlarged in the width direction sothat insertion of the steel form 10 into the binding beam accommodatingportion 2 b is facilitated and the space between the steel form 10 andthe binding beam accommodating portion 2 b may be filled with wood orthe like after the insertion of the steel form 10. After the steel form10 is installed as described above, the steel form 10 is supported bymeans of a temporary support so as to be capable of enduring thesubsequent concrete placement.

Subsequently, the main bar arrangement, deck plate installation, andplacement steps are performed as illustrated in FIG. 8(b).

The main bars 30 are arranged in the steel form 10 in the main bararrangement step. Specifically, the main bars 30 are assembled, liftedby means of a heavy machine or the like, and dropped into and disposedin the groove portion. Likewise, the main bars 30 (not illustrated) ofthe girder 2 are dropped into and disposed in the wooden form 2 a of thegirder 2. Then, the main bars 30 of the binding beam 1 are bent in, forexample, end portions and fixed to the main bars 30 of the girder 2.

The deck plates 3 are installed on the flange portions 14 of the steelform 10 in the deck plate installation step. In the deck plateinstallation step, the plurality of deck plates 3 are placed on theflange portions 14 so as to bridge one binding beam 1 and anotheradjacent binding beam 1 and fixed to the flange portions 14 by boltfastening or the like.

In the placement step, the binding beam concrete 20 is placed in thegroove portion that is configured by the pair of side plate portions 13and the bottom plate portion 12 of the steel form 10 installed in thesteel form installation step. Specifically, in this placement step,concrete is poured into the groove portion of the steel form 10 while avibrator is used for air bubble mixing prevention. As described above,in Embodiment 1, concrete is simultaneously placed in the wooden form 2a of the girder 2 and on the deck plate 3, and then the binding beam 1,the girder 2, and the slab are integrally formed.

Subsequently, the penetration step is performed as illustrated in FIG.8(c). Formed in the penetration step is the web opening 40 penetratingthe steel form 10 installed in the steel form installation step and thebinding beam concrete 20 placed in the placement step. Specifically, inthis penetration step, the side plate portion 13 of one Z-steel 11, thebinding beam concrete 20, and the side plate portion 13 of the otherZ-steel 11 are sequentially penetrated by means of an excavator (such asa known drill) after the concrete placed in the placement step realizesa predetermined strength, and the web opening 40 is formed as a result.Then, the plurality of web openings 40 are formed by similar work beingperformed in a plurality of places of the beam. The number of the webopenings 40 may correspond to the number of ducts to be disposed.

The size and the position of disposition of the web opening 40 can bedetermined similarly to general RC. For example, it is preferable thatthe maximum diameter of the web opening 40 is one-third or less of theheight of the binding beam 1 (dimension D in FIG. 3), the position ofdisposition is other than the end portion of the binding beam 1 (rangeto one-tenth of the total length of the binding beam 1 and rangeequivalent to twice the diameter of the web opening 40 from the endportion of the binding beam 1), and the interval between the pluralityof web openings 40 is at least 1.5 times the total value of therespective diameters of the web openings 40. The size and the positionof disposition of the web opening 40 are not limited to the example andcan be determined in any manner insofar as the required strength of thebinding beam 1 can be ensured.

Lastly, a duct is passed through the web opening 40 formed in thepenetration step. The passage of the duct (not illustrated) is performedby a known method and will not be described in detail. This is the endof the description of the binding beam construction method according toEmbodiment 1.

Effects of Embodiment 1

As described above, in the binding beam 1 of Embodiment 1, since thesteel form 10 having the groove portion can be formed by joining thepair of Z-steels 11 to each other on the joining surface of the bottomplate portion 12, roll molding or press molding of a thin steel platefor groove portion formation can be omitted and the labor and costrequired for the processing can be reduced. Also, the groove portion canbe formed by joining the pair of Z-steels 11 to each other at aconstruction site, and thus the pair of Z-steels 11 can be stacked andtransported in a state where the frame members are not joined to eachother. As a result, the number of the Z-steels 11 that can betransported at one time can be increased and the labor and cost requiredfor the transport can be reduced.

In addition, it is possible to calculate a complex allowable bendingmoment and a complex allowable shear force taking the respective bearingratios of the steel form 10 and the binding beam concrete 20 intoaccount and it is possible to optimize the design of the binding beam 1.

Since the bottom plate portions 12 are joined to each other in a statewhere they are overlapped at the joining surface, when the binding beamconcrete 20 is cast on the steel form 10, leakage of the binding beamconcrete 20 from the joint portion can be suppressed, and constructionis improved. Further, by directly joining the bottom plate portions 12each other, another plate or the like for connecting the bottom plateportions 12 to each other becomes unnecessary, and the cost required forjoining can be reduced.

Since the flange portion 14 is provided, the load of the slab supportedby the binding beam 1 can be received by the flange portion 14 and isallowed to smoot flow to the binding beam 1 and the proof stress of thebinding beam 1 is improved.

Since the reinforcing portion 15 is provided at the outer end of theflange portion 14, buckling of the flange portion 14 at a time when thebinding beam concrete 20 is placed on the groove portion or the flangeportion 14 of the steel form 10 can be suppressed by the reinforcingportion 15 and the proof stress of the binding beam 1 is improved.

Embodiment 2

Next, a binding beam according to Embodiment 2 will be described.Schematically, Embodiment 2 relates to a construction method in which acylindrical form is pre-installed in the web opening forming portion anda web opening is formed in the place of cylindrical form installation bypost-concrete placement cylindrical form removal. The configuration ofthe binding beam according to Embodiment 2 after completion issubstantially the same as the configuration of the binding beamaccording to Embodiment 1, and regarding the configuration substantiallythe same as the configuration of Embodiment 1, the same referencenumerals and/or names as those used in Embodiment 1 are attached theretoas necessary, and a description thereof will be omitted. The followingdescription covers a steel form forming method and a binding beamconstruction method in relation to the binding beam according toEmbodiment 2. Description will be appropriately omitted as to proceduressimilar to those of Embodiment 1.

Steel Form Forming Method

First, an example of the method for forming the steel form 10 accordingto Embodiment 2 will be described. First, the Z-steel 11 is manufacturedat a factory. At this time, a circular hole 51 is formed in advance at aposition corresponding to the web opening forming portion in the Z-steel11. In other words, in Embodiment 2, the circular hole 51 is provided ateach of the positions six places in total in the drawing) in the sideplate portion 13 of the Z-steel 11 that corresponds to the web opening40 illustrated in FIG. 1(a) by means of any tool such as a cuttingmachine. Subsequently, the Z-steel 11 having the circular holes 51 asdescribed above is transported to a construction site, and then a pairof the Z-steels 11 transported to the construction site are bolt-joinedtogether. The steel form 10 is formed as a result. The specific methodfor the joining is similar to that of Embodiment 1 and will not bedescribed in detail.

Binding Beam Construction Method

The method for constructing a binding beam 50 according to Embodiment 2will be described below. FIG. 9 is a set of cross-sectional perspectiveviews corresponding to the A-A arrow cross section in FIG. 1(a). FIG.9(a) illustrates the binding beam 50 at the completion of steel forminstallation and cylindrical form installation steps. FIG. 9(b)illustrates the binding beam 50 at the completion of main bararrangement, deck plate installation, and placement steps. FIG. 9(c)illustrates the binding beam 50 at the completion of a penetration step.

First, the steel form installation and cylindrical form installationsteps are performed as illustrated in FIG. 9(a). The steel forminstallation step is similar to that of Embodiment 1 and will not bedescribed in detail.

In the cylindrical form installation step, a cylindrical form 52 isinserted into the circular hole 51 formed in the steel form 10. Theaxial length of the cylindrical form 52 (length in the direction)exceeds the width of the groove portion of the steel form 10 (length inthe +X-X direction), and thus both end portions of the cylindrical form52 protrude to the outside from the circular hole 51 as illustrated inthe drawing. Although the cylindrical form 52 may be hollow or solid andany material can be used for the cylindrical form 52 insofar as the loadof concrete can be withstood, the case of a solid wooden form will bedescribed below. After the cylindrical form 52 is installed as describedabove, the gap between the outer periphery of the cylindrical form 52and the inner periphery of the circular hole 51 is filled with a sealingmaterial (not illustrated) such as putty. Concrete leakage is deterredas a result.

Subsequently, the main bar arrangement, deck plate installation, andplacement steps are performed as illustrated in FIG. 9(b). The main bararrangement, deck plate installation, and placement steps can be carriedout similarly to the main bar arrangement, deck plate installation, andplacement steps according to Embodiment 1, respectively. Accordingly,detailed descriptions of the steps will be omitted.

Subsequently, the penetration step is performed as illustrated in FIG.9(c). Formed in the penetration step is the web opening 40 penetratingthe steel form 10 installed in the steel form installation step and theconcrete placed in the placement step. Specifically, in this penetrationstep, the cylindrical form 52 installed in the cylindrical forminstallation step is removed to the outside of the binding beam 50 afterthe concrete placed in the placement step realizes a predeterminedstrength. As a result, the web opening 40 is formed at the positionwhere the cylindrical form 52 was present (web opening forming portion).In a case where the cylindrical form 52 is given a hollow shape, a ductcan be inserted through the hollow part of the steel form 10, and thusthe cylindrical form 52 may not be removed. In addition, a part of theduct may be used as the cylindrical form 52.

Lastly, a duct is passed through the web opening 40 formed in thepenetration step. The passage of the duct (not illustrated) is performedby a known method and will not be described in detail. This is the endof the description of the method for constructing the binding beam 50according to Embodiment 2.

Effects of Embodiment 2

As described above, with the binding beam 50 of Embodiment 2, it ispossible to form the web opening 40 simply by removing the cylindricalform 52 Accordingly, it is possible to simplify the work for forming theweb opening 40 at a construction site.

[III] Modification Examples Regarding Embodiments

The embodiments according to the invention have been described. However,the specific configurations and means of the invention can be modifiedand improved in any manner within the scope of the technical idea ofeach invention described in the claims. Such modification examples willbe described below.

Regarding Problems to be Solved and Effects of Invention

First of all, the problems to be solved by the invention and the effectsof the invention are not limited to the above and may vary with thedetails of the implementation environment and configuration of theinvention, and only some of the problems described above may be solvedand only some of the effects described above may be achieved in somecases.

Inter-Embodiment Relationship

The features of each embodiment and the features according to each ofthe following modification examples may be mutually replaced or onefeature may be added to another. For example, the web opening 40 may beformed by the method according to Embodiment 1 (with a drill or thelike) at the position in the binding beam 50 where the web opening 40 isnot formed after the binding beam 50 is formed by the method accordingto Embodiment 2 (by pre-disposition of the cylindrical form 52 in theweb opening forming portion).

Regarding Dimensions and Materials

The dimension, shape, material, ratio, and the like of each portion ofthe binding beams 1 and 50 described in the detailed description of theinvention and the drawings are merely examples, and any otherdimensions, shapes, materials, ratios, and the like can be used as well.For example, the front-view angle that is formed by the side plateportion 13 and the bottom plate portion 12, the front-view angle that isformed by the side plate portion 13 and the flange portion 14, and thefront-view angle that is formed by the flange portion 14 and thereinforcing portion 15 may be obtuse angles or acute angles althougheach of the angles is a right angle in each of the embodiments asillustrated in FIG. 1(b).

FIG. 10 is a set of views illustrating a state where the Z-steel 11 istransported. FIG. 10(a) is an end view illustrating the state oftransport of the Z-steel 11 of Embodiment 1. FIG. 10(b) is an end viewillustrating the state of transport of a Z-steel 11′ according to afirst modification example. In a state where the plurality of Z-steels11 of Embodiment 1 are overlapped as illustrated in FIG. 10(a), H(hereinafter, referred to as first overlap dimension) an intervalbetween one of straight lines connecting a plurality of outermostportions on one side of the Z-steel 11 and the straight line that isparallel to the straight line and passes through the outermost portionof the Z-steel 11 on the other side. As illustrated in FIG. 10(b), theZ-steel 11′ in which each of the angle formed by the side plate portion13 and the bottom plate portion 12 and the angle formed by the sideplate portion 13 and the flange portion 14 is an obtuse angle is assumedas the Z-steel 11′ according to the first modification example, and in astate where a plurality of the Z-steels 11′ are overlapped, H′(hereinafter, referred to as second overlap dimension) is an intervalcorresponding to the first overlap dimension H. The second overlapdimension H′ is smaller than the first overlap dimension H. Accordingly,transport efficiency improvement can be achieved by the Z-steel 11′being formed as in FIG. 10(b).

FIG. 1 is a set of views illustrating the steel form 10 according to asecond modification example. FIG. 11(a) is a plan view of the steel form10 that is yet to be bent. FIG. 11(b) is a side view of the steel form10 that is bent. The pre-bending steel form 10 may be formed as one flatsteel plate 60 as illustrated in FIG. 11(a). In the steel plate 60, eachof a boundary line L1 between the side plate portion 13 and the bottomplate portion 12, a boundary line L2 between the side plate portion 13and the flange portion 14, and a boundary line L3 between the flangeportion 14 and the reinforcing portion 15 has a slit. The steel form. 10that is illustrated in FIG. 11(b) can be formed by bending each portionof the steel plate 60 in the slit by means of a known device or thelike. In this case, the steel form 10 may be, for example, transportedas the flat steel plate 60 in FIG. 11(a). Accordingly, the overlapdimension of the steel form 10 in the state of transport decreases andtransportation efficiency improvement can be achieved.

Alternatively, the steel form 10 may be divided in one or more places inthe longitudinal direction and joined at an installation site. Theposition and place of division of the steel form 10 can be determined inany manner. For example, the steel form 10 may be divided into aplurality of units capable of being loaded on a transport vehicle interms of length. It is preferable that the position of division is aplace where the moment that is applied to the post-joining steel form 10is small. Any joining method is applicable to the steel form 10 afterthe division. For example, a pair of the steel forms 10 brought in touchwith each other in the divided state may be connected via a connectionplate (not illustrated) provided on the outside surfaces of the sideplate portions 13 of the pair of steel forms 10. A drill screw, a bolt,or the like can be used for fixing of the connection plate to the sideplate portion 13. In addition, when the binding beam concrete 20 isplaced in the post-joining steel form 10, it is preferable to supportthe steel form 10 by using a temporary support at the joining point ofthe steel form 10. By the divided structure being adopted as describedabove, the manufacturing workability and the transport efficiency of thesteel form 10 can be improved. In addition, even the binding beam 1 thathas a large span can be built by joining of a plurality of thestandard-span binding beams 1.

Regarding Girder Joining Portion

Although a case where the girder 2 is a reinforced concrete beam hasbeen described in each embodiment, the invention is not limited theretoand the girder 2 may be, for example, a steel-framed beam. FIG. 12 is aset of views illustrating the vicinity of the joining portion between abinding beam 100 and a girder 110 according to a third modificationexample. FIG. 12(a) is a right side view and. FIG. 12(b) is across-sectional view taken along arrow B-B in FIG. 12(a). As illustratedin FIG. 12. in the third modification example, the end portion of thebinding beam 100 in the axial center direction (+Y-Y direction) isjoined to the girder 110, which is a steel-framed beam. Here, a dustpanshaped member (dustpan member) 120 having a substantially U-shaped XZcross section is joined by welding or the like to the side surface ofthe girder 110. The binding beam 100 and the girder 110 can be joinedtogether by the steel form 10 of the binding beam 100 being accommodatedin the dustpan shaped member 120.

Alternatively, the swallowing width of the binding beam 1 in the girder110 may be further increased. FIG. 13 is a set of views illustrating thevicinity of the joining portion between the binding beam 1 and thegirder 110 according to a fourth modification example. FIG. 13(a) is aright side view and FIG. 13(b) is a plan view As illustrated in FIG. 13,the girder 110 is configured as reinforced concrete and disposed in thegirder 110 are a plurality of the main bars 30 disposed along thelongitudinal direction of the girder 110 and a rib 31 disposed in adirection orthogonal to the longitudinal direction and surrounding theplurality of main bars 30 (in FIG. 13(b), only the outermost main bars30 in the Y direction are illustrated among the main bars 30 forconvenience of illustration). A notch 111 for causing the girder 110 toswallow the tip of the binding beam 1 is formed in the place in the sideportion of the girder 110 that corresponds to the binding beam 1. Thebinding beam 1 is disposed so as to be orthogonal to the girder 110 andjoined in part to the girder 110 via the notch 111. Specifically, thepair of side plate portions 13 of the binding beam 1 is accommodated inthe girder 1 by a length L10, which is equal to or greater than thecover thickness of the girder 110, beyond the side surface of the girderon the binding beam 1 side whereas the bottom plate portion 12, theflange portion 14, and the reinforcing portion 15 of the binding beam 1stay at a position where the end surface on the girder 110 side issubstantially flush with the side surface of girder on the binding beam1 side. Here, the “cover thickness” is the thickness part of concretethat reaches the rib 31 from the side surface of the girder 110 and isthe thickness of a dimension L11 in FIG. 13. It is possible to furtherimprove the joining strength of the binding beam 1 and the girder 110 bythe girder 110 accommodating the binding beam 1 to the extent of thelength L10, which is equal to or greater than the cover thickness Lil ofthe girder 110, as described above.

In the example illustrated in FIG. 13, in particular, hairpin bars 17are swallowed by the girder 110. The hairpin bars 17 are a plurality ofrod-shaped bar arrangements arranged side by side along the X direction.For the pair of side plate portions 13 swallowed by the girder 110 to beconnected to each other, the hairpin bars 17 are passed through thearrangement holes (see reference numeral 13 a in FIG. 16 to be describedlater) formed in the pair of side plate portions 13 and fixed by weldingor the like to the pair of side plate portions 13. When the hairpin bar17 is disposed at a position closer to the Y-direction middle positionof the girder 110 than the rib 31 (position on the -Y direction side),in particular, the hairpin bar 17 and the pair of side plate portions 13surround the rib 31 at least in part. in this structure, a movement ofthe hairpin bar 17 in the ±Y direction is regulated by the rib 31, andthus it is possible to further improve the joining strength of thebinding beam 1 and the girder 110 by means of the bearing pressure ofthe hairpin bar 17 (local compressive force). In the example illustratedin FIG. 13, it is assumed that in the pair of side plate portions 13,only the height part that is minimum required for disposition of arequired number of the hairpin bars 17 (three in FIG. 13) isaccommodated in the girder 110. Accordingly, the unnecessary height parthas a notch 18 formed therein and notched. The binding beam 1 can beaccommodated in the girder 110 by any method. For example, concreteplacement may be performed on the steel form 10 and the form of thegirder 110 in a state where the end portion of the steel form 10 isaccommodated in the form of the girder 110 via the notch portion 111formed in the form of the girder 110 and the hairpin bar 17 is disposedto surround the rib 31 at least in part and fixed to the side plateportion 13.

Alternatively, the pair of side plate portions 13 may be simplyaccommodated in the girder 110 with the height as it is and without thenotch 18 being provided. FIG. 14 is a right side view illustrating thevicinity of the joining portion between the binding beam 1 and thegirder 110 according to a fifth modification example (in the fifthmodification example and sixth to eighth modification examples, placeswithout description are similar to those of the fourth modificationexample). As illustrated in FIG. 14, in the binding beam 1, the pair ofside plate portions 13 extend toward the girder 110 with the height asit is and the pair of side plate portions 13 are accommodated in thegirder 110 to the extent of a length that is equal to or greater thanthe cover thickness of the girder 110.

Alternatively, a part of the pair of side plate portions 13 and abearing pressure effective part may be swallowed by the girder 110. FIG.15 is a right side view illustrating the vicinity of the joining portionbetween the binding beam 1 and the girder 110 according to the sixthmodification example. FIG. 16 is a perspective view of an end portion ofthe steel form 10 of the binding beam 1 in FIG. 15. As illustrated inFIGS. 15 and 16, in the binding beam 1. the pair of side plate portions13 extend toward the girder 110 with the height as it is (or a part ofthe bottom plate portion 12 is notched along with a part of thereinforcing portion 15 and the flange portion 14 of the steel form 10)and the pair of side plate portions 13 are accommodated in the girder11.0 to the extent of the length L10, which is equal to or greater thanthe cover thickness of the girder 110. In this structure, a part of thebinding beam 1 accommodated in the girder 110 needs to be provided witha part receiving the bearing pressure of the hairpin bar 17 (bearingpressure effective part). The bearing pressure effective part may varywith the desired bearing pressure. For example, the width of the bearingpressure effective part is set to approximately 100 mm (=sum of anX-direction width L12, 50 mm, of a part of the flange portion 14 leftwithout being cut and an X-direction width L13, 50 mm, of a part of thebottom plate portion 12 left without being cut). When the bearingpressure effective part has such a width, the possibility ofinterference with the rib 31 is low, and thus smooth swallowing into thegirder 110 is possible.

Alternatively, the part to be swallowed in the girder 110 may beretrofitted. FIG. 17 is a right side view illustrating the vicinity ofthe joining portion between the binding beam 1 and the girder 110according to the seventh modification example. As illustrated in FIG.17, the pair of side plate portions 13 in addition to the bottom plateportion 12, the flange portion 14, and the reinforcing portion 15 of thebinding beam 1 has an end surface on the girder 110 side staying at aposition substantially flush with the side surface of the girder 110 onthe binding beam 1 side. Here, a joining plate 19 is fixed, by anymethod including a drill screw and a bolt, to the outside surfaces ofthe pair of side plate portions 13 and only the joining plate 19 isaccommodated in the girder 110 by the length L11, which is equal to orgreater than the cover thickness of the girder 110, beyond the sidesurface of the girder 110 on the binding beam 1 side. In this structure,it is not necessary to perform processing such as providing of a notchfor the steel form 10 that has a complicated shape and the joining plate19 has only to be retrofitted in the side plate portion 13, which leadsto easy construction.

The binding beams 1 disposed on both sides of the girder 110 may beconnected to each other. FIG. 18 is a side view illustrating thevicinity of the joining portion between each binding beam 1 and thegirder 110 according to the eighth modification example. FIG. 19 is aplan view of FIG. 18. As illustrated in FIGS. 18 and 19, provided onboth sides of the girder 110 are the pair of binding beams 1 disposedalong a direction orthogonal to the longitudinal direction of the girder110 and the pair of binding beams 1 are disposed at positions on thesame straight line that correspond to each other and brought in touchwith the girder 110. The pair of binding beams 1 are connected to eachother via a hairpin bar 17′ fixed from above to the flange 14. Even in acase where a tensile force is applied to the binding beam 1 in adirection away from the girder 110, the hairpin bar 17′ in thisstructure is capable of countering the tensile force.

In each of the embodiments, the binding beam concrete 20 and the girderconcrete are placed at the same time. However, the invention is notlimited thereto and the binding beam concrete 20 and the girder concretemay be placed one by one. In a case where the girder concrete is placedfirst, for example, the side surface of the solidified girder concretemay be chipped into a shape (hat shape) substantially corresponding tothe axial cross-sectional shape of the binding beams 1 and 50 and thebinding beam concrete 20 may be placed after the end portion of thesteel form 10 of each of the binding beams 1 and 50 is installed at thechipped part.

Regarding Flange Portion

Although the flange portion 14 is provided in each embodiment, theflange portion 14 may be omitted and the steel form 10 may be configuredas a member having a substantially U-shaped axial cross section.Although the flange portion 14 is provided at the upper end of the sideplate portion 13, the invention is not limited thereto and the flangeportion 14 may be provided at a position other than the upper end (suchas a position that is below the upper end by a predetermined distance(such as several centimeters)).

Regarding Reinforcing Portion

Although the reinforcing portion 15 is provided at the outer end of theflange portion 14 in each embodiment, the reinforcing portion 15 may beomitted in a case where the flange portion 14 is capable of enduring theload of concrete. In addition, reinforcing means for further reinforcingthe flange portion 14 may be provided in addition to or instead of thereinforcing portion 15. For example, reinforcement may be performed bymeans of a reinforcing steel plate affixed to the upper surface or thelower surface of the flange portion 14. The steel plate may be affixedthrough the forward-rearward direction of the flange portion 14 or maybe intensively affixed only to a part particularly requiring proofstress (such as the vicinity of the middle of the flange portion 14 inthe forward-rearward direction).

Alternatively, the shape of the reinforcing portion 15 may be changed.FIG. 20 is a cross-sectional view corresponding to the A-A arrow crosssection in FIG. 1(a) and is a cross-sectional view of a steel form 210of a binding beam 200 according to a ninth modification example. Asillustrated in FIG. 20, the steel form 210 is provided with a secondreinforcing portion 216. The second reinforcing portion 216 is a steelplate extending from the lower end of a reinforcing portion 215 toward aside plate portion 213. By the second reinforcing portion 216 beingprovided as described above, the local buckling of the outer end of theflange portion 14 that pertains to a case where the slab concrete 4 isplaced and a flange portion 214 receives the load of a slab can be moreeffectively deterred. In addition, it is possible to reduce the overallthickness of the steel form 210 by locally reinforcing only alow-strength part by means of the second reinforcing portion 216.

The second reinforcing portion 216 can be provided in another aspect aswell. FIG. 21 is a cross-sectional view corresponding to the A-A arrowcross section in FIG. 1(a) and is a cross-sectional view of the steelform 210 of the binding beam 200 according to a tenth modificationexample. In the example illustrated in FIG. 21, the second reinforcingportion 216 is formed by the outer end of the flange portion 214 beingfolded back toward the side plate portion 213 and the reinforcingportion 215 is omitted.

Regarding Z-steel

In each embodiment, the pair of Z-steels 11 are overlapped with eachother and bolt-joined. Specific methods for the joining are not limitedthereto. FIG. 22 is a set of cross-sectional views corresponding to theA-A arrow cross section in FIG. 1(a). FIG. 22(a) is a cross-sectionalview of the steel form 210 of the binding beam 200 according to aneleventh modification example. FIG. 22(b) is a cross-sectional view of asteel form 310 of a binding beam 300 according to a twelfth modificationexample. In other words, as illustrated in FIG. 22(a), the surfaces ofbottom plate portions 221 of a pair of Z-steels 220 that are brought intouch with each other may be used as joining surfaces 222 and thesurfaces may be joined by welding. Alternatively, as illustrated in FIG.22(b), end portions of bottom plate portions 321 of a pair of Z-steels320 may be folded back upward, the inside surface of this folded part322 may be combined as a joining surface 323, and the folded part may bejoined by means of a caulking fitting 324 in that state. Alternatively,drill screw or screw driving may be performed from below or above on theend portions of the bottom plate portions 321 of the pair of Z-steels320 so that the end portions are joined to each other. In this case, thedrill screw or the screw may be allowed to protrude by, for example,approximately several centimeters into the inner spaces of the pair ofZ-steels 220 so that the joining strength between the Z-steel 220 andthe binding beam concrete 20 placed in the inner space is furtherenhanced.

Regarding Non-opening Member

A point that has been described in each embodiment is that a temporarymember (member removed before concrete placement) such as a batten and aU-shaped veneer board is provided for fixing of the relative positionsof the pair of Z-steels 11 during the formation of the steel form 10(mutual joining of the pair of Z-steels 11). A permanent member forfixing the relative positions of the pair of Z-steels 11 (memberembedded without pre-concrete placement removal, hereinafter, referredto as non-opening member) may be provided instead of or in addition tothe temporary member. FIG. 23 is a set of cross-sectional viewscorresponding to the A-A arrow cross section in FIG. 1(a). FIG. 23(a)illustrates a steel form 410 of a binding beam 400 according to athirteenth modification example. FIG. 23(b) illustrates a steel form 510of a binding beam 500 according to a fourteenth modification example. inother words, a non-opening member 422 may be provided for connectionbetween flange portions 421 of a pair of Z-steels 420 as illustrated inFIG. 23(a) or a non-opening member 522 may be provided for connectionbetween side plate portions 521 of a pair of Z-steels 520 as illustratedin FIG. 23(b). When the relative positions of the pairs of Z-steels 420and 520 are fixed by means of the non-opening members 422 and 522, it ispossible to prevent the pairs of Z-steels 420 and 520 from mutuallyopening outward due to the weight of the binding beam concrete 20 afterthe placement of the binding beam concrete 20.

The non-opening member 522 illustrated in FIG. 23(b), in particular, ispreferably provided in the range (range of the dimension L12 in FIG.23(b)) from the upper end positions of the pair of side plate portionsto the position that is below the upper end positions by one-third ofthe height of the pair of side plate portions. in a case where the pairsof Z-steels 420 and 520 are likely to mutually open outward, the sideplate portion 13 is likely to pivot to the outside with the boundarybetween the bottom plate portion 12 and the side plate portion 13 as afulcrum, and thus the distance between the pair of side plate portions13 tends to increase as the upper end of the side plate portion 13 isapproached. By the non-opening member 522 being provided in theabove-described range, however, the relative positions of the pair ofside plate portions 13 can be fixed at a position relatively close tothe upper ends of the pair of side plate portions 13, and thus themutual outward opening of the pair of side plate portions 13 can be moreeffectively prevented than in a case where the non-opening member 522 isprovided at a position below the range.

Regarding Main Bar Arrangement Step

In each embodiment, the main bar arrangement step is performed after thesteel form installation step. However, the invention is not limitedthereto and the steel form installation step may be performed after themain bar arrangement step. At this time, the main bar 30 is disposedfirst in the main bar arrangement step, the pair of Z-steels 11 aredisposed so as to cover the main bar 30 from below, and in a state wherethe bottom plate portions 12 of the pair of Z-steels 11 overlap eachother, a bolt being inserted from below through the bottom plate portion12 and thereby the pair of Z-steels 11 may be joined to each other.

One embodiment of the present invention provides a steel form, which isa steel form for steel-framed concrete beam formation, includes a pairof frame members, wherein each of the pair of frame members is providedwith a bottom plate portion and a side plate portion extending upwardfrom the bottom plate portion, the bottom plate portion has a joiningsurface for joining the respective bottom plate portions of the pair offrame members to each other, and a groove portion allowing concreteplacement is formed by the respective bottom and side plate portions ofthe pair of frame members.

According to this embodiment, since the steel form having the grooveportion can be formed by joining the pair of frame members to each otheron the joining surface of the bottom plate portion, roll molding orpress molding of a thin steel plate for groove portion formation can beomitted and the labor and cost required for the processing can bereduced. Also, the groove portion can be formed by joining the pair offrame members to each other at a construction site, and thus the pair offrame members can be stacked and transported in a state where the framemembers are not joined to each other. As a result, the number of theframe members that can be transported at one time can be increased andthe labor and cost required for the transport can be reduced.

Another embodiment of the present invention provides the steel formaccording to the above embodiment, wherein an allowable bending momentor an allowable shear three of the steel-framed concrete beam iscalculated by Equation (1) below: (Equation 1) F_(a)=F_(RC)+β·F_(S)wherein, F_(a): an allowable bending moment or an allowable shear forceof the steel-framed concrete beam, F_(RC): an allowable bending momentor an allowable shear force of the concrete, β: a burden factor of anallowable bending moment or an allowable shear force of the steel form,which is 0.5 or less, and Fs: an allowable bending moment or anallowable shear force of the steel form.

According to this embodiment, it is possible to calculate a complexallowable bending moment and a complex allowable shear force taking therespective bearing ratios of the steel form and the concrete intoaccount and it is possible to optimize the design of the steel-framedconcrete beam.

Another embodiment of the present invention provides the steel formaccording to the above embodiment, wherein a part of the steel-framedconcrete beam is joined to a girder, and the steel form is provided withan end portion on the girder side in a longitudinal direction of thesteel form, accommodated in the girder via a notch formed in a sidesurface of the girder, and having a length equal to or greater than acover thickness of the girder.

According to this embodiment, it is possible to further improve thejoining strength of a binding beam and a girder by the girderaccommodating the binding beam to the extent of the length, which isequal to or greater than the cover thickness of the girder.

Another embodiment of the present invention provides the steel formaccording to the above embodiment, wherein a non-opening member forfixing the pair of side plate portions to each other is provided in arange from an upper end position of the pair of side plate portions to aposition below the upper end position by one-third of a height of thepair of side plate portions.

According to this embodiment, since relative positions of the pair ofside plate portions can be fixed at a position relatively close to theupper end position of the pair of side plate portions, mutual outwardopening of the pair of side plate portions can be more effectivelyprevented than in a case where the non-opening member is provided at aposition below this range.

Another embodiment of the present invention provides the steel formaccording to the above embodiment, wherein the steel form is providedwith a flange portion extending outward from an upper end of the sideplate portion.

According to this embodiment, since the flange portion is provided, loadof a slab supported by the steel-framed concrete beam can be received bythe flange portion and is allowed to smoothly flow to the steel-framedconcrete beam and proof stress of the steel-framed concrete beam isimproved.

Another embodiment of the present invention provides the steel formaccording to the above embodiment, wherein the steel form is providedwith a reinforcing portion extending downward or upward from an outerend of the flange portion.

According to this embodiment, since the reinforcing portion is providedat the outer end of the flange portion, buckling of the flange portionat a time when the concrete is placed on the groove portion or theflange portion of the steel form can be suppressed by the reinforcingportion and the proof stress of the steel-framed concrete beam isimproved.

REFERENCE SIGNS LIST

-   1 Binding beam-   2 Girder-   2 a Wooden form-   2 b Binding beam accommodating portion-   2 c Flange accommodating portion-   2 d Sealing material-   3 Deck plate-   4 Slab concrete-   10 Steel form-   11, 11′ Z-steel-   12 Bottom plate portion-   13 Side plate portion-   13 a Arrangement hole-   14 Flange portion-   15 Reinforcing portion-   16 Joining surface-   17, 17′ Hairpin bar-   18 Notch-   19 Joining plate-   20 Binding beam concrete-   30 Main bar-   31 Rib-   40 Web opening-   50 Binding beam-   51 Circular hole-   52 Cylindrical form-   60 Steel plate-   100 Binding beam-   110 Girder-   111 Notch-   120 Dustpan shaped member-   200 Binding beam-   210 Steel form-   213 Side plate portion-   214 Flange portion-   215 Reinforcing portion-   216 Second reinforcing portion-   220 Z-steel-   221 Bottom plate portion-   222 Joining surface-   300 Binding beam-   310 Steel form-   320 Z-steel-   321 Bottom plate portion-   322 Folded part-   323 Joining surface-   324 Caulking fitting-   400 Binding beam-   410 Steel form-   420 Z-steel-   421 Flange portion-   422 Non-opening member-   500 Binding beam-   510 Steel form p0 520 Z-steel-   521 Side plate portion-   522 Non-opening member

1. A steel form for steel-framed concrete beam formation, comprising: apair of frame members, wherein each of the pair of frame members isprovided with a bottom plate portion and a side plate portion extendingupward from the bottom plate portion, the bottom plate portion has ajoining surface for joining the respective bottom plate portions of thepair of frame members to each other, and a groove portion allowingconcrete placement is formed by the respective bottom and side plateportions of the pair of frame members.
 2. The steel form according toclaim 1, wherein an allowable bending moment or an allowable shear forceof the steel-framed concrete beam is calculated by Equation (1) below:F _(a) =F _(RC) +β·F _(S)   (Equation 1) wherein, F_(a): an allowablebending moment or an allowable shear force of the steel-framed concretebeam, F_(RC): an allowable bending moment or an allowable shear force ofthe concrete, β: a burden factor of an allowable bending moment or anallowable shear force of the steel form, which is 0.5 or less, andF_(S): an allowable bending moment or an allowable shear force of thesteel form.
 3. The steel form according to claim 1, wherein a part ofthe steel-framed concrete beam is joined to a girder, and the steel formis provided with an end portion on the girder side in a longitudinaldirection of the steel form, accommodated in the girder via, a notchformed in a side surface of the girder, and having a length equal to orgreater than a cover thickness of the girder.
 4. The steel formaccording to claim 1, wherein a non-opening member for fixing the pairof side plate portions to each other is provided in a range from anupper end position of the pair of side plate portions to a positionbelow the upper end position by one-third of a height of the pair ofside plate portions.
 5. The steel form according to claim 1, wherein thesteel form is provided with a flange portion extending outward from anupper end of the side plate portion.
 6. The steel form according toclaim 5, wherein the steel form is provided with a reinforcing portionextending downward or upward from an outer end of the flange portion.